Germanium tin oxide thin films for uncooled infrared detectors

ABSTRACT

Microbolometer is a class of infrared detector whose resistance changes when the temperature changes. In this work, we deposited and characterized Germanium Oxide thin films mixed with Sn (Ge—Sn—O) for uncooled infrared detection. Ge—Sn—O were deposited by co-sputtering of Sn and Ge targets in the Ar+O environment using a radio frequency sputtering system. Optical characterization shows that the absorption in Ge—Sn—O was most sensitive in the wavelength ranges between 1.0-3.0 μm. The transmission data was further used to determine the optical energy band gap (0.678 eV) of the thin-film using Tauc&#39;s equation. We also found the variations of absorption coefficient (1.4802×10 5  m- 1 -1.0097×10 7  m −1 ), refractive index (1.242-1.350), and the extinction coefficient (0.3255-8.010) for the wavelength ranges between 1.0-3.0 μm. The thin film&#39;s resistivity measured by the four point probe was found to be 4.55 Ω-cm and TCR was in the range of −2.56-−2.25 (%/K) in the temperature range 292-312K. In light of these results it can be shown that this thin film is in keeping with the current standards while also being more cost and time effective.

1. TECHNICAL FIELD OR FIELD OF INVENTION

This invention relates to the Germanium Tin Oxide (Ge—Sn—O) thin filmsfor a microbolometer and a method of manufacturing thereof and aninfrared detector or microbolometer using Ge—Sn—O. It reports thedeposition, characterization and properties of Ge—Sn—O thin films fortheir electrical, optical, mechanical and morphological properties formicrobolometer's sensing layer. Microbolometers are the infrared sensorswhich change their resistances when the temperature changes. Themicrobolometer operates in Midwave Infrared (MWIR, 3 to 5 μm) andLongwave Infrared (LWIR, 8 to 14 μm) wavelengths for detecting infrared(IR) radiation. The unique properties of Ge—Sn—O will be used foruncooled IR detection.

2. BACKGROUND OF THE INVENTION

Thermal IR detectors are heated by the incident IR radiation and providedetection through the change in a measurable parameter. For these typesof detectors, wavelengths of interest are mainly in the atmosphericwindows—ranging from 3 to 5 (MWIR) and 8 to 14 (LWIR) μm wavelengthranges, due to the high transmission through atmospheric air of morethan 80% and peak IR emission of room temperature bodies is at 9-10 μmof wavelengths. Thermal detectors like microbolometers are being usedcontact-less temperature measurement, night vision cameras for defense,security and surveillance applications, search and rescue and many otherthermal imaging applications because of their low-cost, betterperformance and compact size. A main factor in dictating how well athermal detector will work is the detector's responsivity. Responsivityis the ability of the device to convert the incoming radiation into anelectrical signal. Detector material properties influence this value;therefore, several main material properties are investigated whichinclude temperature coefficient of resistance (TCR), optical bandgap,tansmittance, reflectance and absorptance and resistivity in thewavelengths of interest. Other properties such as compatibility withcomplementary metal oxide semiconductor (CMOS) processing technology,low cost and reliability and stability of the material while exposed toinfrared radiation are important.

The microbolometer's sensing materials are classified in two maincategories—metals and semiconductors. Metals such as Ti, Ni or Ni—Fealloys had been reported as bolometer's sensing layers.

Amorphous Si (a-Si) and Vanadium Oxide (VO_(x)) are two of the mostwidely used materials for sensing layers of microbolometers These twomaterials suffer from low TCR and low absorption which yields lowerfigures of merits such as responsivity, detectivity, and noiseequivalent temperature difference. By using various atomic compositionsof Germanium, Tin, and Oxide in Ge—Sn—O thin films, this inventionreports using Ge—Sn—O thin films for microbolometer's sensing layer.

For microbolometers made of semiconducting sensing layers(semiconducting microbolometers), thermal change on a material with ahigh TCR causes a change in electrical resistance, thus allowing ameasurable parameter across the detector with heating and cooling. Oncethe microbolometer's sensing layer's temperature changes, there arethree possible mechanisms of heat loss. First, heat is lost throughconduction/convection through the atmosphere surrounding the detectorthermometer, that is described below. This is minimized by vacuumpackaging the detectors. Today, it has been a common practice to includethe wafer-level vacuum packaging scheme for all commercially availablemicrobolometers. Second, heat is lost through radiation. The selectionof materials will impact this mechanism. However, the materials that arepreferred for low heat loss also absorb less infrared radiation, whichis not desirable. This mechanism represents the ultimate limit on theperformance of the detector. Third, the heat is lost through thermalconduction through the supporting structure of the thermometer. Thedesign of the supporting structure will minimize the thermal conductanceof the structure.

The performance of an IR detector for imaging is most commonly describedby a parameter called noise-equivalent temperature difference (NETD),which is the measurement of how well a thermal detector will distinguishbetween slight differences in thermal radiation in the image. The bestcryogenically cooled quantum devices will have NETD values below 20 mK.Although no thermal uncooled detector has reached such low values, thetheoretical limits of thermal IR detectors operating at ambienttemperature are close to the values of cooled quantum detectors forwavelengths above 8 μm. Since thermal noise power increases as thesquare root of heat conduction, the heat conduction to the environmentposes the largest limit in terms of detection. To lower this limit, thethermal bridges between detector to substrate and housing must beminimized. Also, lowering the heat capacity of the detector element byreducing the thickness of the detector structure leads to a largetemperature change per radiation input, further reducing the effect ofnoise. Commercial microbolometers with a lens of an f-number equal to 1has an NETD value of 35 mK. Micromachining has allowed furtherimprovement of thermal detectors, with the most advanced IR Focal PlaneArray (FPA) currently based on microbolometers of vanadium oxide andamorphous silicon, achieving NETD between 25 and 50 mK.

For a thermal detector, the sensitive element is referred to as thethermometer. The thermometer is typically thermally isolated from thesubstrate to improve the responsivity by suspending it above thesubstrate using micromachining techniques. The performance of a thermaldetector depends upon the thermal capacity C_(th), the rate at whichthermal energy is lost through the thermal conductance of the structure,G_(th), and the radiative thermal conductance, G_(rad). The radiativethermal conductance for a gray body, assuming the emissivity is equal tothe absorptivity, is given by equation G_(rad)=4ησAT³; where η is theaverage absorption of the detector, a is the Stefan-Boltzmann constant,A is the surface area and T is the absolute temperature. Theconductive/convective loss is neglected since the detector is typicallyoperated in vacuum. The temperature change due to a sinusoidallymodulated photon flux is given by:

$\begin{matrix}{{\Delta\; T} = \frac{\eta\Phi}{{G_{eff}\left( {1 + {\omega^{2}\tau_{th}^{2}}} \right)}^{1/2}}} & (1)\end{matrix}$

Where, Φ is the radiant energy flux, is the angular modulation frequencyof the incident radiation, and is the thermal time constant(C_(th)/G_(th)). The effective thermal conductance, G_(eff), is obtainedthrough a heat balance and is given by:G _(eff) =G _(th) +G _(rad) ±αP _(bias)  (2)

Where, α is the TCR of the thermometer, P_(bias) is the power dissipatedin the bias of the detector. The sign of the power bias term dependsupon the type of bias. The “+” sign corresponds to the voltage bias casewhile the “−” sign corresponds to the current bias case. For the case ofa semiconductive microbolometer, the TCR is negative, which means thatthe power dissipated in the detector effectively increases the effectivethermal conductance G_(eff).

There are other figures of merits than NETD for microbolometers whichare described below:

Temperature Coefficient of Resistance (TCR):

TCR exhibits how rapidly the resistance of the sensing material respondsto a change in temperature and is expressed as

$\begin{matrix}{\alpha = {{\frac{1}{R} \cdot \frac{d\; R}{d\; T}} = {{\frac{1}{R}\frac{\Delta\; R}{\Delta\; T}} = {- \frac{E_{a}}{{kT}^{2}}}}}} & (3)\end{matrix}$Here, E_(a) is the activation energy and k is the Boltzmann constant.TCR is a material property, so the higher the value, the better it isfor IR uncooled detection.

Responsivity: Responsivity is a measure of the dependence of the signaloutput of a detector upon the input radiant power. The detector outputsignal may be current or voltage. Thus the voltage responsivity, R_(v),is defined as the detector output voltage per unit of detector inputpower.

$\begin{matrix}{R_{v} = \frac{{\eta\alpha}\;{RI}_{b}}{{G_{th}\left( {1 + {\omega^{2}\tau^{2}}} \right)}^{1/2}}} & (4)\end{matrix}$

Where, η, I_(b), G_(th), ω, and τ are the absorption coefficient, biascurrent, thermal conductance, angular frequency, and time constant ofthe device, respectively. The first three terms of the numerator in theright-hand side of equation (4) (η, α and R) depend on materialproperties of the microbolometer. Voltage responsivity is expressed inV/W while current responsivity is expressed in A/W. The voltageresponsivity of the bolometer is increased by decreasing the thermalconductance of the structure. The thermal time constant of themicrobolometer is in the millisecond range as it involves the thermalmass of the sensing layer which needs to heat up for change inresistance because of IR radiation.

Detectivity:

Detectivity, D*, is the area normalized signal to noise ratio. It hasthe unit of cmHz^(1/2)/W. The detectivity is expressed by

$\begin{matrix}{D^{*} = \frac{R_{v}\sqrt{A_{d}\Delta\; f}}{\Delta\; v_{n}}} & (5)\end{matrix}$where, Δv_(n) is the total noise voltage observed in the electricalbandwidth Δf and is the sum of noises from the sensing element of themicrobolometer—Johnson noise, random telegraph switching noise,1/f-noise, generation and recombination noise. Higher responsivityrepresents higher detectivity.

Table 1 shows the list of materials used as the sensing layer ofmicrobolometer infrared detector. The main drawback found in all thematerials is the low TCR which also ends up in lower responsivity anddetectivity when they are used in microbolomter.

TABLE I TCR and other figures of merit of bolometer sensing materials.Pixel TCR Detectivity Responsivity Resistivity size Material (%/K)(cmHz^(1/2)/W) (V/W) (Ω-cm) (μm²) V₂O₅ 2.8   6 × 10⁵ 36 1.7 200 × 800V_(0.95)W_(0.05) 4.10   1 × 10⁹ N/A 40 N/A a-SiGe −2 N/A N/A ~40 N/APoly-SiGe −1.91  8.3 × 10⁸ 15,000 N/A N/A poly-SiGe −2 2.31 × 10⁹  1.4 ×10⁴ N/A 25 × 25 (CVD deposited) a-Si:H 2.8-3.9 N/A   1 × 10⁶ N/A 48 × 48a-Ge_(x)Si_(1-x)O_(y) −2.27 − 8.27 × 10⁶ 1.05 × 10⁴ 4.22 × 10² − 40 × 40−8.69 3.47 × 10⁹ Y-Ba-Cu-O 4.02  1.6 × 10⁹  3.8 × 10⁵ 32.42 7000 × 10000

In current invention, we observed that the absorption in the thin filmwas most sensitive in the wavelength ranges of 1.0-3.0 μm range. Theoptical energy band gap (0.678 eV) of the thin-film was using Tauc'sequation. In addition to these, we also found the variations ofabsorption coefficient (1.4802×10⁵ m-¹-1.0097×10⁷ m⁻¹), refractive index(1.242-1.350), and the extinction coefficient (0.3255-8.010) for thewavelength ranges between 1.0-3.0 μm. We found the thin film'sResistivity to be 4.55 Ω-cm by four point probe method.

3. SUMMARY OF THE INVENTION

The present invention had been focused on to overcome the problemsmentioned above to obtain high TCR and responsivity using Ge—Sn—O thinfilms for microbolomters by a simplified process and a method thereofand a microbolemter using the Ge—Sn—O thin films. The microbolometerencompasses a change in resistance on the sensing material due to theabsorption of heat flux on it, causing a change across the sensingmaterial's resistance to be measured across the electrodes.

The second objective of this invention to prepare a stable Ge—Sn—O thinfilms for uncooled infrared detection with high reproducibility propertyand a method for manufacturing thereof and an infrared detector usingthe Ge—Sn—O thin films.

Third objective of this invention is to provide Ge—Sn—O thin films whichwill increase the sensitivity of microbolometer and a method ofmanufacturing thereof using Ge—Sn—O thin films.

Fourth objective of this invention is to prepare Ge—Sn—O thin films forbolometer's sensing layer to reduce the overall fabrication cost of thedevice by providing a simplier process and method of manufacturingthereof, and a microbolometer using the Ge—Sn—O thin films.

According to the invention, the thin films of Ge_(x)Sn_(y)O_(z) compriseof Ge, Sn, and O elements with values of 0.39≤x≤0.54, 0.09≤y≤0.20,0.19≤z≤0.46.

In accordance with the invention, the thin films of Ge—Sn—O aredeposited by co-sputtering of Sn and Ge targets in the Ar+O environmentusing a radio frequency sputtering system. The films are deposited onsilicon or cover glass or other conformal substrates. The thin filmsalso deposited by the chemical vapor deposition process using variousgases and precursors.

According to the invention, the thin films of Ge_(x)Sn_(y)O_(z) has athickness ranging from 1400 nm to 1450 nm where 0.39≤x≤0.54,0.09≤y≤0.20, 0.19≤z≤0.46.

The atomic composition of the thin films of Ge_(x)Sn_(y)O_(z) where0.39≤x≤0.54, 0.09≤y≤0.20, 0.19≤z≤0.46 are controlled by changing thepower of the DC sputter, power of the radio frequency sputter, processpressure, ratio of the gases' flow during the deposition and substratetemperature during the deposition.

According to the invention, there is an infrared detector comprising ofbolometer. The bolometer comprises the thin films of Ge_(x)Sn_(y)O_(z)as the sensing layer which has Ge, Sn, and O elements with values of0.39≤x≤0.54, 0.09≤y≤0.20, 0.19≤z≤0.46.

As the sensing layer of microbolometer, the thin films of Ge—Sn—O has athickness ranging from 1400 nm to 1450 nm.

As described above, in the present invention, the Ge—Sn—O film for abolometer is prepared by a simplified process having a high TCR valueand a low noise value.

Further, in the present invention, stable and high reproducibilityproperties are obtained.

According to the invention since the Ge—Sn—O films are manufacturedusing cheap equipment and a simplified method, the cost of fabricatingthe microbolometer device will be reduced.

The new invention will increase the sensitivity of the microbolometerdevices using Ge—Sn—O films.

4. BRIEF DESCRIPTION OF FIGURES

FIG. 1: Flowchart illustrating the method for preparing theGe_(x)Sn_(y)O_(z) thin films where 0.39≤x≤0.54, 0.09≤y≤0.20,0.19≤z≤0.46.

FIG. 2: Atomic Composition of Ge—Sn—O thin films determined by energydispersive spectroscopy at 5 keV.

FIG. 3: Refractive index and extinction coefficient of Ge—Sn—O thinfilm.

FIG. 4: Transmittance, reflectance and absorptance of Ge—Sn—O thin film.

FIG. 5: Optical band gap of Ge—Si—Sn—O determined from transmittance,reflectance and absorptance data.

FIG. 6: Arrhenius plot at various temperature to determine theactivation energy of Ge—Sn—O thin films.

FIG. 7: Variations of resistivity and TCR with temperature.

FIG. 8: Cross section of a Ge—Sn—O Microbolometer.

5. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

We present here the microbolometer elements and methods for forming thesame. The sensing layer of microbolometer is fabricated using variousatomic compositions of Germanium, Tin, and Oxygen to form an alloy ofgermanium-tin-oxide (Ge—Sn—O). The variation in atomic composition inthe alloy of Ge—Sn—O will allow the materials' properties to be varied.The variation of the atomic composition in Ge—Sn—O alloy will vary thefundamental material properties such as activation energy and carriermobility. This way the key microbolometer device figures of merits suchas resistivity, TCR, responsivity, absorption in IR region of interest,noise, detectivity, noise equivalent temperature difference are variedand optimized for better device performance.

Variation and optimization of atomic composition in the alloy of Ge—Sn—Oallow us to increase the room temperature TCR to a value which issignificantly higher than the current reported values (from −2%/k to−5%/k). TCR is one of the important properties of microbolometer and itdefines the sensitivity of the microbolometer device which is defined aschange in electrical resistance of the device with change intemperature. The greater the value of TCR, the better will be thesensitivity of the microbolometer device. Herein, we used oxygen to bondwith other semiconductors (Ge and Sn) to increase the TCR among otherproperties. In the preferred embodiment we found the TCR varied between−2.25%/K to −2.56%/K for the temperature ranges 292K-312K. Theresistivity for this temperature range for the thin films of Ge—Sn—O isvaried between 0.5426 Ohm-cm to 45.67 Ohm-(cm). The higher resistivityof the Ge_(x)Sn_(y)O_(z) where 0.39≤x≤0.54, 0.09≤y≤0.20, 0.19≤z≤0.46,alloy had been resulted from lower electrical conductivity which arisesbecause of the low carrier mobility associated with this.

The atomic composition of the microbolometer sensing layer made ofGe_(x)Sn_(y)O_(z) thin film layer consists of two semiconductingmaterials (Ge, Sn) forming an alloy and bonded with oxygen where0.39≤x≤0.54, 0.09≤y≤0.20, 0.19≤z≤0.46. The element Ge, may or may not bedoped with impurities such as n-type or p-type impurities while they areused as one of the constituent elements. In one embodiment, we disclosethe atomic composition of Ge_(x)Sn_(y)O_(z) where the value of “x” isthe atomic percentage of Ge in that alloy relative to the atomicpercentage of Sn. The value of “x” varies between 0.39 to 0.54 while thevalue of “y” varies between 0.09 to 0.20. The value of “z” variesbetween 0.19 to 0.46.

In another embodiment, the thin film of Ge_(x)Sn_(y)O_(z) will bedeposited using the sputtering process, where 0.39≤x≤0.54, 0.09≤y≤0.20,0.19≤z≤0.46. The sputtering process consists of radio frequency as wellas direct current method.

In the present invention, the exemplary embodiments are described indetails in the related art work hereinafter. The drawings presentedhere, will explain the current invention better although it should benoted that the present invention is not limited to the drawings.

FIG. 1 is a flow chart illustrating a method for fabricating theGe_(x)Sn_(y)O_(z) thin film to be used as the sensing layer of themicrobolometer in accordance with an embodiment of the present inventionwhere 0.39≤x≤0.54, 0.09≤y≤0.20, 0.19≤z≤0.46. FIG. 1 shows the method formanufacturing a Ge_(x)Sn_(y)O_(z) thin film for a bolometer's sensinglayer where 0.40≤x≤0.90, 0.39≤x≤0.54, 0.09≤y≤0.20, 0.19≤z≤0.46. Thepreferred embodiment of the present invention includes is completed in asingle step: this includes the deposition of Ge_(x)Sn_(y)O_(z) thin filmon a substrate; In accordance with the invention, the thin films ofGe_(x)Sn_(y)O_(z) are deposited by co-sputtering of Sn and Ge targets inthe Ar+O environment using a radio frequency sputtering system. Thefilms are deposited on silicon or cover glass or other conformalsubstrates. The thin films are also deposited by DC sputtering ormetal-organic chemical vapor deposition (MOCVD) or chemical vapordeposition process using Ge, Sn, and O based gases and precursors.

In this case, the DC sputter and the RF sputter use argon (Ar) ornitrogen (N₂) plasma. The thickness of the Ge_(x)Sn_(y)O_(z) thin filmmay ranges from 1400 nm to 1450 nm. According to the invention, the thinfilms of Ge_(x)Sn_(y)O_(z) comprise of Ge, Sn, and O elements withvalues of where 0.39≤x≤0.54, 0.09≤y≤0.20, 0.19≤z≤0.46. When the z ofGe_(x)Sn_(y)O_(z) is >0.46, the electric resistance of the thin film istoo large. Accordingly, it is not suitable for the bolometerapplications. When the oxygen component, z, is close to zero, theelectric resistance is too small and the TCR value is small as well,thereby making it not suitable for the bolometer.

A turbo pump evacuated the chamber to a base pressure of 3×10⁻⁷ Torr orless before sputtering. This compound target of Ge and Tin (Sn) targetboth were used simultaneously to deposit the Ge_(x)Sn_(y)O_(z) thinfilms on silicon or glass or other substrates which may be flexible orrigid. The deposition will be done by using DC and/or RF sputter. The DCsputter and RF sputter use Argon (Ar) or nitrogen (N₂) plasma. Thedeposition process takes place in an oxygen atmosphere. The sputteringwill take as long as 2 hours but in this case, the Ge and Sn targetswere sputtered for 40 minutes. Ge was sputtered at 250 W while Sn wassputtered at 75 W. The preferred embodiment includes a design ofmaterial for each of the layers of the microbolometer whose compositionand thickness leads to high figures of merits. Germanium Tin Oxide(Ge—Sn—O) was created by co-sputtering Ge with Sn targets in the Ar+Oenvironment using a radio frequency sputtering system.

Radio frequency sputtering system is defined as a technique involved inalternating the electrical potential of the current in the vacuumenvironment at radio frequencies to avoid a charge building up oncertain types of sputtering target materials, which over time willresult in arching into the plasma that spews droplets creating qualitycontrol issues on the thin films and will even lead to the completecessation of the sputtering of atoms terminating the process. In thiscase, Ge at 250 W and Sn at 75 W were sputtered for 40 minutes with adeposition rate of 5.997 Å per second on a silicon wafer.

After sputtering process, energy dispersive spectroscopy function from ascanning electron microscope was used to determine the atomiccompositions of the thin film.

FIG. 2: shows the atomic Composition of Ge_(x)Sn_(y)O_(z) thin filmsdetermined by energy dispersive spectroscopy done at 5 keV where0.39≤x≤0.54, 0.09≤y≤0.20, 0.19≤z≤0.46. The unique peaks in FIG. 2indicate the estimated value of abundance of the different elements inthe thin film. In the preferred embodiment, Ge_(x)Sn_(y)O_(z) thin filmsconsist of 30.2% Oxygen, 14.3% Tin, 48.3% Germanium, and 7.1% Carbonwhere 0.39≤x≤0.54, 0.09≤y≤0.20, 0.19≤z≤0.46. As mentioned earlier, thethin films of Ge_(x)Sn_(y)O_(z) comprise of Ge, Sn, and O elements. Inthe preferred embodiment, the values ranges 0.39≤x≤0.54, 0.09≤y≤0.20,0.19≤z≤0.46 Determination of atomic composition of the constituentelements in the thin films of Ge—Sn—O is crucial for not only thecorrect elemental composition in Ge—Sn—O but also for unique behavior ofGe_(x)Sn_(y)O_(z) thin film (where 0.39≤x≤0.54, 0.09≤y≤0.20,0.19≤z≤0.46) for sensing layer of microbolometer.

FIG. 3 indicates the refractive index and extinction coefficient of theGe—Sn—O thin film. Refractive index is a number that describes how lightpropagates through that medium, while the extinction coefficientindicates several different measures of the absorption of light in amedium. Extinction coefficient refers to a measure of the rate ofdecrease in the intensity of electromagnetic radiation (as light) as itpasses through a given substance;

Extinction coefficient k(λ) is given by:

$\begin{matrix}{{k(\lambda)} = \frac{a\;\lambda}{4\;\pi}} & (6)\end{matrix}$

Where a is the absorption coefficient, λ is the wavelength.

Reflective index n(λ) is the measure of the propagation of a ray oflight as it passes from one medium to another and it was also derivedby:

$\begin{matrix}{{n(\lambda)} = {{n_{s}(\lambda)}\left( \frac{1 + \sqrt{R}}{1 - \sqrt{R}} \right)^{0.5}}} & (7)\end{matrix}$Where, n_(s) is the refractive index of the substrate (Silicon wafer inour case) and R is the Reflectance. The refractive index for Ge—Sn—Ovaried between 1.2 to slightly above 1.3 for the wavelength rangesbetween 1.0 μm to 3.0 μm. These values are different from pure Ge, Sn,or their oxides. The extinction coefficient varied between 0.32 to 8.01for the wavelength ranges between 1.0 μm to 3.0 μm. The values of theextinction coefficient are also different from pure Ge, Sn, or theiroxides. The values of extinction coefficient and refractive index atvarious wavelengths determines the absorption, reflection andtransmission through the thin films. The absorption in the film at aparticular wavelength is crucial as because of the absorption the thinfilms' temperature changes and we get a TCR. The values of absorptioncoefficient along with the reflection coefficient and transmittance atvarious wavelength is mentioned in FIG. 4.

FIG. 4 indicates the transmittance, reflectance and absorptance ofGe_(x)Sn_(y)O_(z) thin film, where 0.39≤x≤0.54, 0.09≤y≤0.20,0.19≤z≤0.46. These measurements were carried out using monochromator,infrared (IR) light source, pyroelectric detector, mechanical chopper,dynamic signal analyzer and computer. Infrared (IR) light signal wasgenerated using a IR source which came out of the monochromator and waschopped at 100 Hz. Results of peak signal at 100 Hz and noises wererecorded from dynamic signal analyzer in order to find the correspondingtransmittance and reflectance in the thin films between the wavelengthranges 2.5-5.5 μm. Kirchhoff's law was then used to calculate theabsorptance.

Transmittance τ(λ) through a thin film is expressed as the ratio oftransmitted flux (ϕ_(λt)) to the incident flux (ϕ_(λi))

$\begin{matrix}{{\tau(\lambda)} = \frac{\phi_{\lambda\; t}}{\phi_{\lambda\; i}}} & (8)\end{matrix}$where, λ is the wavelength.

Kirchhoffs law relating to absorptance (α), transmittance (τ) andreflectance (ρ) is expressed as:α+τ+ρ=1  (9)

Absorption coefficient is a measure of the rate of decrease in theintensity of electromagnetic radiation (as light) as it passes through agiven substance. For application as the sensing layer of microbolometer,we need the absorption value to be as high as possible. Higherabsorption in the sensing layer will heat up the material and then theresistance of the material will reduce. So the higher absorption isassociated with greater sensitivity. The addition of Sn inGe_(x)Sn_(y)O_(z) thin films, increase the absorption in the wavelengthranges between 2 to 4.5 where 0.40≤x≤0.90, 0.08≤y≤0.60, 0.01≤z≤0.20.

FIG. 5 is Optical band gap of Ge_(x)Sn_(y)O_(z) determined fromtransmittance, reflectance and absorptance data which shown earlier.Extrapolation of (αhν)^(1/2)=0 of linear portion in the plot of (αhν)versus the photon energy hν, gave the value of optical bandgap asmentioned in FIG. 5 where 0.39≤x≤0.54, 0.09≤y≤0.20, 0.19≤z≤0.46. Todetermine the optical bandgap, we used Tauc's equation for directbandgap semiconductor which is expressed below:αhν=B(hν−E ₉)^(1/2)  (10)Where B is a constant, hν is the photon energy and E_(g) is the energyband gap. (αhν)² was graphed as a function of hν and the energy band gapwas determined at the hν value where α=0. Optical bandgap is animportant parameter for optical detectors such as microbolometers. Theoptical bandgap of Ge—Sn—O in this case is 0.678 eV which is unique forthe Ge_(0.483)Sn_(0.143)O_(0.302) thin film. The optical bandgap playsan important role in optoelectronic properties of the material. When thephoton falls on top of the material, if its energy is higher than theoptical bandgap then it creates and electron hole pair. This is theprinciple of operation for a photon detector. For thermal detectors likemicrobolometers, the detector material's temperature rises and theresistivity fo the material changes. Hence, electrical bandgap is aninherent property of the material—Ge—Sn—O.

FIG. 6 illustrates Arrhenius plot at various temperature to determinethe activation energy of Ge_(x)Sn_(y)O_(z) thin films where 0.39≤x≤0.54,0.09≤y≤0.20, 0.19≤z≤0.46. Activation energy is the minimum energyrequired to cause a process (such as a chemical reaction) to occur. Itis expressed using Arrhenius formula as follows;

$\begin{matrix}{k = {{{- \frac{E_{a}}{R}}\left( \frac{1}{T} \right)} + {\ln\; A}}} & (11)\end{matrix}$where, k represents the rate constant, E_(a) is the activation energy, Ris the gas constant (8.3145 J/K mol), and T is the temperature expressedin Kelvin. A is known as the frequency factor, having units of L mol⁻¹s⁻¹, and takes into account the frequency of reactions and likelihood ofcorrect molecular orientation.

Four point probe instrument was used to measure average resistance ofthe film by passing current through the outside two points of the probeand measuring the voltage across the inside two points. This resistanceis called sheet resistance (R_(s)).

If the spacing between the probe points is constant, and the conductingfilm thickness is less than 40% of the spacing, and the edges of thefilm are more than 4 times the spacing distance from the measurementpoint, the average resistance of the film or the sheet resistance isgiven by;Rs=4.53×V/I  (11)Total resistance R is given by;R=R _(s) *t  (12)

Where, R_(s) is the measured sheet resistance and t is the filmsthickness. From FIG. 6 it is seen that the plot of natural logarithm ofresistivity (ρ) versus 1/kT is a straight line and the slope of thestraight line provides the value of the activation energy associatedwith this process. The higher the activation energy, higher will be theTCR associated with it. Adding oxygen with the Ge—Sn compound increasesthe activation energy (Ea). The activation energy forGe_(0.483)Sn_(0.143)O_(0.302) thin film is 0.1859 eV.

In FIG. 7, we show the variations of temperature coefficient ofresistance, TCR and resistivity for a resistor is determined bymeasuring the resistances values over an appropriate temperature rangein Kelvin. The TCR is calculated as the average slope of the resistancevalue over this interval. TCR exhibits how rapidly the resistance of thesensing material responds to a change in temperature and is expressedas;

$\begin{matrix}{\alpha = {{\frac{1}{R} \cdot \frac{d\; R}{d\; T}} = {{\frac{1}{R}\frac{\Delta\; R}{\Delta\; T}} = {- \frac{E_{a}}{{kT}^{2}}}}}} & (13)\end{matrix}$Here, E_(a) is the activation energy and k is the Boltzmann constant.

TCR is a material property, so the higher the value, the better it isfor IR uncooled detection. The room temperature (301K) TCR value is2.53%/K which is at on par with the Vanadium Oxide that is used as oneof the most popular materials for microbolometers. The resistivity ofthe for Ge_(0.483)Sn_(0.143)O_(0.302) thin film varied between 3.24Ohm-cm to 1.88 Ohm-cm for the temperature range between 290 K to 325 K

FIG. 8 shows the cross sectional view of the microbolometer usingGe—Sn—O sensing layer. The substrate 101 in this case is Silicon. Thedepositions of different layers other than the Al and polyimide weredone by a RF magnetron sputtering system equipped with a turbo pump anda three-inch target holder. Prior to sputtering, the process chamber wasevacuated to 3×10⁻⁷ Torr by the turbo pump. Sputtering was done at 3mTorr pressure. An Ar flow of 54 SCCM was used in every sputteringprocess other than deposition. In fabricating the Ge_(x)Sn_(y)O_(z)bolometer, lift off technique was used for patterning all the filmsbecause of its simplicity where 39≤x≤54, 09≤y≤20, 19≤z≤46.

The fabrication of the bolometer starts by depositing 400 nm of siliconnitride 111 on a cleaned lightly doped p-type, three inch diametersilicon wafer 101. This layer of silicon nitride 111 serves as theelectrical insulation for the substrate and would withstand all thesolvents used in next fabrication steps. Then a 400-nm-thick layer of Allayer 141 was deposited by thermal evaporation and patterned. The cryopump of the evaporator was cooled down to 20 K after which the samplewas mounted. The chamber was evacuated to 1×10⁻⁶ Torr prior toevaporation. A deposition rate of 5 angstrom/sec was achieved at 100Ampere of applied current. To perform the lift off, negative resistNR7-1500P from Futurrex Corporation was spin coated on the wafer at 3000rpm for 30 seconds. Then the wafer was pre baked at 150° C. for 60seconds on a hot plate and exposed under the ultraviolet light. A postexposure bake for 60 seconds was done at 120° C. on a hot plate. Thenthe resist was developed in RD6, a negative resist developer fromFuturrex Corporation, for 50 seconds. The resist thickness was found tobe about 1.8 μm after developing. This was found to be thick enough tolift off 0.4-μm-thick Al layer 141. After depositing Al film, the waferwas kept in 1165 photo resist striper for about two hours to completethe lift off process. The wafer was then rinsed with acetone, methanol,and DI water followed by a blow dry in nitrogen to make it clean. ThisAl layer 141 would serve as a mirror for reflecting the infrared raysand form the basis of an optically resonant cavity.

A sacrificial layer 151 of photo definable polyimide PI-2737 from HDMicrosystems was spin coated, patterned by conventional photolithographyand wet etching process. The polyimide was cured in the convection oven.After curing the polyimide thickness was found 2.2 For patterning, thepolyimide was spin coated at a speed of 1650 rpm for 60 seconds. Then itwas baked on two step hot plate. In first step, it was baked at 70° C.,while in second step it was baked at 100° C., both for three minutes.Then it was exposed in ultraviolet light. For developing polyimide,developer DE 9040 along with rinse solution RI 9180, both from HDMicrosystem, were used. To store these solutions and complete developingprocess, four tanks and one squeeze bottle were used. First two of thefour tanks were filled with 100% DE 9040 solution, third tank was filledwith 50% DE 9040 and 50% RI 9180 solutions, fourth tank was filled with25% DE 9040 and 75% RI 9180 solutions, while the squeeze bottle wasfilled with 100% RI 9180 solution. The wafer was kept inside the firsttwo tanks for 10 seconds and agitated ultrasonically. Then it wastransferred to the second and third tank respectively where it was keptfor 15 seconds in each of them. Then the wafer was rinsed with RI 9180solution for 20 seconds, by holding it vertically to remove all thepolyimide flakes. By using four tanks instead of one tank, the problemof flakes generation was solved. The polyimide thickness at that pointwas about 5.3 μm. This was then cured in an oven at 250° C. for fourhours in nitrogen ambient. The temperature was ramped slowly (from roomtemperature to 250° C. in one and half hours) to avoid possible thermalstress in the film.

To achieve low thermal mass, the sensing layer of Ge_(x)Sn_(y)O_(z) 181was made 0.6579 μm-thick. In order to make the sensing layermechanically strong as well as free of warping, a sandwich structure ofGe_(x)Si_(y)Sn_(1−(x+y))O_(z) was made employing silicon nitride where,39≤x≤54, 09≤y≤20, 19≤z≤46. The sandwich layers increase the thermal massof the detector. However, they were found to be necessary, since thefirst type of the bolometers were fabricated without silicon nitridesandwich layers and they were found to be warped after removing thesacrificial polyimide. To achieve the sandwich structure, first thebottom silicon nitride layer 161 was deposited and patterned. Thethickness of this layer was set to 100 nm. Silicon nitride was chosenfor this case, because silicon nitride is known to passivate silicondioxide, although in current work it was not observed any significanteffect of passivating the Ge_(x)Si_(y)Sn_(1−(x+y))O_(z) sensing layer181 with silicon nitride where 39≤x≤54, 09≤y≤20, 19≤z≤46. Next, a200-nm-thick NiCr (20% Ni, 80% Cr) electrode arm 121 was deposited andpatterned. NiCr has very low thermal conductivity, and thus providesgood thermal isolation between bolometer thermometer and substrate.

To form an Ohmic contact with p-type Ge_(x)Sn_(y)O_(z), a 50-nm-thick Nifilm 171 was deposited on top of NiCr arm and patterned where 39≤x≤54,09≤y≤20, 19≤z≤46. Then, the sensing layer of Ge_(x)Sn_(y)O_(z) wasdeposited in an Ar:O₂ environment from a compound target of Ge and Snwhere 39≤x≤54, 09≤y≤20, 19≤z≤46. Next, a 14-nm-thick NiCr absorber 191and 100-nm-thick silicon nitride 201 layers were deposited. This siliconnitride layer 201 works as the top layer of the sandwich structure.These three layers were lifted of together. Finally, a 300-nm-thick Nibond-pad-layer 131 was deposited on top of NiCr for the simplicity ofbonding the device ultrasonically. At this point, the bolometerfabrication was completed. The sacrificial layer of polyimide under thebolometer was not removed for the sake of simplicity in devicefabrication.

We claim:
 1. A method of forming a thin film comprising:Ge_(x)Sn_(y)O_(z) where, 39≤x≤54, 09≤y≤20, 19≤z≤46.
 2. The method ofclaim 1, wherein the thin film is deposited by one of radio frequencysputtering, direct current sputtering, chemical vapor deposition, metalorganic chemical vapor deposition techniques.
 3. The method of claim 1,wherein the thin film of Ge_(x)Sn_(y)O_(z) where x, y, z values,39≤x≤54, 09≤y≤20, 19≤z≤46 is deposited in Ar:O environment by radiofrequency and direct current sputtering method.
 4. The method of claim1, wherein the transmittance, reflectance and absorptance of the thinfilm varies between 0% to 90%, 0% to 75% and 0% to 100% respectively forthe wavelength ranges 1.0 μm to 3.0 μm.
 5. A thin film comprising:germanium tin oxide with a chemical formula Ge_(x)Sn_(y)O_(z) with x, y,z values 39≤x≤54, 09≤y≤20, 19≤z≤46.
 6. The thin film of claim 5, whoserefraction index varies between 1.2 to 14 and extinction coefficientvaries between 0.32 to 8.01 for the wavelength ranges from 1.0 μm to 3.0μm.
 7. The thin film of claim 5, whose transmittance, reflectance andabsorptance varies between 0% to 90%, 0% to 75% and 0% to 100%respectively for the wavelength ranges 1.0 μm to 3.0 μm.
 8. The thinfilm of claim 5, wherein the optical bandgap of the thin film variesbetween 0.51 eV to 0.93 eV.
 9. The thin film of claim 5, wherein theactivation energy of the thin film has a value between 0.007 eV to 0.196eV depending on the atomic composition of the thin film.
 10. The thinfilm of claim 5, wherein the resistivity of the thin film varies between3.24 Ohm-cm to 1.88 Ohm-cm between the temperature ranges 292K-312K. 11.The thin film of claim 5, wherein coefficient of Resistance (TCR) variesin the range—2.56%/K-−2.25%/K between the temperatures ranges 292K-312K.12. A method of forming a microbolometer comprising: a thin filmcomprising Ge_(x)Sn_(y)O_(z) where x, y, z values, 39≤x≤54, 09≤y≤20,19≤z≤46.
 13. The method of claim 12, the thin film is deposited by oneof radio frequency sputtering, direct current sputtering, chemical vapordeposition, metal organic chemical vapor deposition techniques.
 14. Themethod of claim 12, wherein the thin film of Ge_(x)Sn_(y)O_(z) depositedin Ar:O environment by radio frequency sputtering method.
 15. The methodof claim 12, wherein the thin film has a refraction index varies between1.2 to 1.4 and extinction coefficient varies between 0.32 to 8.01 forthe wavelength ranges 1.0 μm to 3.0 μm.
 16. The method of claim 12,wherein the optical bandgap of the thin film varies between 0.51 eV to0.93 eV.
 17. The method of claim 12, wherein the activation energy ofthe thin film has a value between 0.007 eV to 0.196 eV depending on theatomic composition of the thin films.
 18. The method of claim 12,wherein the resistivity of the thin film varies between 3.24 Ohm-cm to1.88 Ohm-cm between the temperature ranges 292K-312K.
 19. The method ofclaim 12, wherein the temperature coefficient of Resistance (TCR) of thethin film varies in the range −2.56%/K-−2.25%/K between the temperaturesranges 292K-312K.